The invention relates to a power switch arrangement having a semiconductor power switch connected by its load path serially in a load circuit, and a clamping circuit, which is connected between a control electrode terminal and a load-side electrode terminal of the semiconductor power switch and, upon turn-off of the semiconductor power switch, sets a clamping voltage across the load path thereof.
In order to relatively rapidly switch off inductive loads with integrated semiconductor power switches, use is made of a clamping circuit which limits the output voltage, that is to say the voltage dropped across the load path, e.g. of a power switching transistor, to a maximum value Vc, which lies below the maximum technology voltage (for example, 60 V). During switch-off operation, a high power loss is converted in the power switching transistor, said power loss depending on the supply voltage, for example a battery voltage, and the energy stored in the inductance of the load. The so-called maximum clamping energy, in other words that energy which can be converted in the power switching transistor without leading to the destruction thereof, is an essential parameter in the specification and should be as large as possible. Said clamping energy depends on the semiconductor technology used, the cooling conditions and the area of the power switching transistor. With advancing miniaturization of the components, the size of the power transistor is determined more and more often by the clamping energy and not by the resistance.
In power switch arrangements that have been customary hitherto, the output voltage during the turn-off operation has been limited to a constant value Vds(t)=constant. The accompanying FIG. 1 shows a customary power switch arrangement that is configured as a low-side power switch. The load path D-S of a power transistor 10, which in this case is an N-channel MOSFET, for example, is connected in a load circuit in series with a load symbolized by an inductance L and a resistance component R. With the supply voltage VB, the voltage Vc is dropped across the load path D-S of the power transistor 10 in the turned-off, that is to say opened, state thereof. As mentioned, a clamping circuit is provided in power switch arrangements of this type, said clamping circuit being designated by the reference numeral 11 in FIG. 1 and limiting the voltage dropped across the load path D-S of the power transistor 10 in the turn-off case, that is to say with power transistor 10 opened, to a constant value. What is disadvantageous is that this solution is not the optimum with regard to the energy consumption capability of the power transistor.
Investigations have shown that the destruction of the power transistor, in the case of excessively high energy, is brought about by a maximum permissible peak temperature of the power transistor being exceeded or by repeated excessively large temperature swing.
In view of the abovementioned disadvantages of the known power switch arrangement, an improved power switch arrangement and an improved turn-off method for turn-off of an inductive load, so that the energy consumption capability of the semiconductor power switch can be increased, would be an improvement in the art.